Method for receiving control information in orthogonal frequency division multiplexing system of mobile communication system

ABSTRACT

The present invention relates to receiving control information in an orthogonal frequency division multiplexing (OFDM) system of a mobile communication system. The present invention includes receiving information related to a number of OFDM symbols in a subframe for receiving first control information, receiving information related to a number of OFDM symbols in the subframe for receiving second control information, decoding the first control information according to the received information related to the number of OFDM symbols in the subframe for receiving the first control information, and decoding the second control information according to the received information related to the number of OFDM symbols in the subframe for receiving the second control information, wherein the number of OFDM symbols for receiving the first control information is less than or equal to the number of OFDM symbols for receiving the second control information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/942,968, filed on Nov. 9, 2010, currently pending, which is acontinuation of U.S. patent application Ser. No. 12/143,647, filed Jun.20, 2008, currently pending, which claims the benefit of earlier filingdate and right of priority to Korean Application No. 10-2007-0122985,filed on Nov. 29, 2007, and U.S. Provisional Application Ser. Nos.60/945,585, filed on Jun. 21, 2007, and 60/946,400, filed on Jun. 27,2007, the contents of which are hereby incorporated by reference hereinin their entirety.

FIELD OF THE INVENTION

The present invention relates to a mobile communication system, and moreparticularly, to a method for receiving control information in anorthogonal frequency division multiplexing system of the mobilecommunication system.

BACKGROUND OF THE INVENTION

In a cellular orthogonal frequency division multiplexing (OFDM) radiopacket communication system, uplink and downlink data packettransmissions are transmitted via a subframe unit. A subframe is definedas a predetermined time period including a plurality of OFDM symbols.Currently, various control information for uplink/downlink data packettransmissions are also transmitted. Such control information includesinformation necessary for transmitting and receiving the uplink/downlinkdata packets, such as radio resource information used for transmittingand receiving the uplink/downlink data packets, a coding scheme, and amodulation scheme, for example. The control information is transmittedusing at least one of the plurality of OFDM symbols included in thesubframe.

A plurality of mobile terminals may communicate through one base stationin a cellular OFDM radio packet communication system. Accordingly,scheduling for allocating radio resources for each of the plurality ofmobile terminals is required. In particular, for a downlink controlchannel transmission, control information for the plurality of mobileterminals may be transmitted together. Thus, scheduling for allocatingradio resources for the control information transmission is alsorequired. Therefore, such scheduling information is also transmitted.

Among the plurality of OFDM symbols included in the subframe, the numberof OFDM symbols used in transmitting the control information and/or thescheduling information may be varied per subframe according to acommunication environment, the amount of control channel information,and the amount of scheduling information, etc. Thus, such informationshould be informed to a receiver. If errors occur in receiving thecontrol information and the scheduling information, it is quite probablethat errors occur in receiving the data of the corresponding subframe.Accordingly, what is needed is a system that overcomes the deficienciesof the prior art, such that control information and schedulinginformation can be decoded with a high success rate.

SUMMARY OF THE INVENTION

The present invention is directed to a method for receiving controlinformation in an orthogonal frequency division multiplexing system of amobile communication system.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the presentinvention is embodied in a method for receiving control information inan orthogonal frequency division multiplexing (OFDM) system of a mobilecommunication system, the method comprising receiving informationrelated to a number of OFDM symbols in a subframe for receiving firstcontrol information, receiving information related to a number of OFDMsymbols in the subframe for receiving second control information,decoding the first control information according to the receivedinformation related to the number of OFDM symbols in the subframe forreceiving the first control information, and decoding the second controlinformation according to the received information related to the numberof OFDM symbols in the subframe for receiving the second controlinformation, wherein the number of OFDM symbols for receiving the firstcontrol information is less than or equal to the number of OFDM symbolsfor receiving the second control information.

Preferably, the second control information is not decoded if the numberof OFDM symbols for receiving the first control information is greaterthan the number of OFDM symbols for receiving the second controlinformation.

In one aspect of the invention, the method further comprises decodingthe second control information using all possible numbers of OFDMsymbols in the subframe for receiving the second control information ifthe number of OFDM symbols for receiving the first control informationis greater than the number of OFDM symbols for receiving the secondcontrol information. In another aspect of the invention, the methodfurther comprises decoding the second control information using allpossible numbers of OFDM symbols in the subframe for receiving thesecond control information greater than or equal to the number of OFDMsymbols for receiving the first control information if the number ofOFDM symbols for receiving the first control information is greater thanthe number of OFDM symbols for receiving the second control information.

Preferably, the first control information comprises an ACK/NACK signaland the second control information comprises a physical downlink controlchannel. Preferably, the information related to the number of OFDMsymbols in the subframe for receiving the first control information isreceived via a broadcast channel. Preferably, the information related tothe number of OFDM symbols in the subframe for receiving the secondcontrol information is received via a physical control channel formatindicator channel. Preferably, the number of OFDM symbols in thesubframe for receiving the second control information is 1, 2 or 3.

In accordance with another embodiment of the present invention, a methodfor transmitting control information in an orthogonal frequency divisionmultiplexing (OFDM) system of a mobile communication system comprisestransmitting information related to a number of OFDM symbols in asubframe for transmitting first control information, transmittinginformation related to a number of OFDM symbols in the subframe fortransmitting second control information, transmitting the first controlinformation according to the transmitted information related to thenumber of OFDM symbols in the subframe for transmitting the firstcontrol information, and transmitting the second control informationaccording to the transmitted information related to the number of OFDMsymbols in the subframe for transmitting the second control information,wherein the number of OFDM symbols for transmitting the first controlinformation is less than or equal to the number of OFDM symbols fortransmitting the second control information.

Preferably, the first control information comprises an ACK/NACK signaland the second control information comprises a physical downlink controlchannel.

Preferably, the information related to the number of OFDM symbols in thesubframe for transmitting the first control information is transmittedvia a broadcast channel. Preferably, the information related to thenumber of OFDM symbols in the subframe for transmitting the secondcontrol information is transmitted via a physical control channel formatindicator channel. Preferably, the number of OFDM symbols fortransmitting the second control information is 1, 2 or 3.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. Features, elements, and aspects of the invention that arereferenced by the same numerals in different figures represent the same,equivalent, or similar features, elements, or aspects in accordance withone or more embodiments.

FIG. 1 is a diagram relatively comparing a varying period of the numberof OFDM symbols through which an ACK/NAK channel is transmitted (m) witha varying period of the number of OFDM symbols for control channeltransmission (n) in accordance with one embodiment of the presentinvention.

FIG. 2 is a diagram illustrating one example of a method for allocatingthe transmission of OFDM symbols of a control channel and an ACK/NAKchannel in an orthogonal frequency division multiplexing (OFDM) systemin accordance with one embodiment of the present invention.

FIG. 3 is a flow chart illustrating one example of a method fortransmitting information on the number of OFDM symbols for controlchannel transmission (n) and a control channel from a base station inaccordance with one embodiment of the present invention.

FIG. 4 is a flow chart illustrating one example of a method forreceiving information on the number of OFDM symbols for control channeltransmission (n) and a control channel in a mobile terminal inaccordance with one embodiment of the present invention.

FIG. 5 illustrates a block diagram of a mobile terminal in accordancewith the present invention.

FIG. 6 is a diagram explaining an example of a method for receivinginformation of OFDM symbols of a downlink control channel in anorthogonal frequency division multiplexing (OFDM) system in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to receiving control information in OFDMsystem of a mobile communication system.

Hereinafter, the present invention will be described in more detail withreference to the accompanying drawings. The detailed descriptiondescribed below with reference to the accompanying drawings intends toexplain exemplary embodiments rather than a sole embodiment where thepresent invention can be carried out. The detailed description describedbelow includes specific details for assisting in a completeunderstanding of the present invention. However, those skilled in theart may appreciate that the present invention can be carried out withoutsuch specific details of the present invention. For example, althoughthe detailed description described below is explained centering oncertain terms, it is not necessarily limited to the terms but the samemeanings can be represented thereby even in the case where it isexplained by optional terms.

In some cases, the present invention may omit a publicly known structureor apparatus in order to avoid obscurity of the present invention, andthe present invention may be presented via a block view and/or a flowchart centering on the core function of each structure and/or apparatus.Also, like reference numerals refer to like elements throughout thespecification.

The below embodiments are the embodiments in which the constituents ofthe present invention and the properties are coupled to each other in apredetermined shape. Each constituent or property should be selectivelyconsidered so far as there are not any specific mentions thereof. Eachconstituent or property may be carried out in a shape that they are notcoupled to another constituent or property. Also, the embodiments of thepresent invention may be configured by combining some constituentsand/or properties. The order of the operations explained in theembodiments of the present invention may be changed. Some constitutionor property of any embodiment may be included in another embodiment, ormay be replaced by the constitution or property corresponding to anotherembodiment.

The embodiments of the present invention are explained centering on adata transmitting/receiving relationship between a base station and amobile terminal. Herein, the base station is a terminal node of anetwork directly performing a communication with the mobile terminal.The specific operation explained to be performed by the base station maybe performed by an upper node of the base station according tocircumstances. In other words, various operations performed forcommunication with the mobile terminal in a network configured of aplurality of network nodes including the base station may be performedby the base station or another network node other than the base station.The “base station” may be replaced by terms, such as fixed station, NodeB, enode B, eNB, and access point, for example. Also, the “mobileterminal” may be replaced by terms, such as User Equipment (UE), MobileStation (MS), and Mobile Subscriber Station (MSS), for example.

When transmitting packet data in a mobile communication system, areceiving side may notify a transmitting side whether or not thereceiving side has received a packet successfully. For example, whenpacket reception is successful, the receiving side may transmit an ACKsignal to inform the transmitting side of the successful reception,therefore allowing the transmitting side to transmit a new packet. Whenpacket reception fails, the receiving side may transmit a NAK signal tothe transmitting side to inform the transmitting side of the failedreception. Accordingly, the transmitting side may retransmit the packetto the receiving side.

The operation described above may be referred to as an automatic repeatrequest (ARQ) operation. An expansion of the ARQ operation may bereferred to as a Hybrid ARQ (HARQ) operation, which is capable ofraising the efficiency of an entire system. The HARQ operation lowerserror probability by combining a retransmission packet with an originalpacket, and by being coupled with a channel coding scheme. In order toimprove performance by applying the HARQ scheme, the HARQ prefers promptACK/NAK responses from a receiver as compared to the previous ARQoperation. Therefore, in the HARQ, the ACK/NAK signal may be transmittedin a physical channel signaling manner.

Preferably, downlink ACK/NAK signals, which are a response to datatransmitted in the uplink, may be transmitted through “m” number of OFDMsymbols of each subframe. Furthermore, it is preferable that the ACK/NAKsignals be transmitted through a part of resource elements within the“m” number of OFDM symbols rather than the entire “m” number of OFDMsymbols. Herein, for example, the “m” value is a value that may varyaccording to a degree of cell coverage. Hereinafter, a method fortransmitting an ACK/NAK channel through which the ACK/NAK signals aretransmitted, and a method for determining OFDM symbols for controlchannel transmission, will be described in more detail.

FIG. 1 is a diagram relatively comparing a varying period of the numberof OFDM symbols through which ACK/NAK channels are transmitted (m) witha varying period of the number of OFDM symbols for control channeltransmission (n) in accordance with one embodiment of the presentinvention. Hereinafter, the embodiments of the present invention will bedescribed for a case where first n OFDM symbols among OFDM symbols inone subframe of a downlink transmission time interval (TTI) unit of anOFDM system (e.g., a 3GPP LTE OFDM radio communication system) are usedfor transmitting uplink/downlink scheduling signals and other controlsignals.

In accordance with the present invention, “n” represents the number ofOFDM symbols used for control channel transmission. A maximum number ofOFDM symbols for control channel transmission is denoted by the value“N”. The “n” value may vary per subframe according to the amount ofuplink/downlink control signals and/or the amount of scheduling signalsto be transmitted to the uplink. For example, if N=3, then n may bedetermined by a natural number less than or equal to 3 (n N, where N=3).

As described above, because the “n” value may vary per subframe, thebase station transmits a control channel format indicator (CCFI)indicating information associated with the “n” value through a physicalcontrol channel format indicator channel (PCFICH) to inform the mobileterminals of the “n” value in each subframe. For example, the CCFI maybe transmitted through a first OFDM symbol of the subframe.

As described above, the “m” value, which is the number of OFDM symbolsthrough which the ACK/NAK channel is transmitted, may also vary.However, the number of OFDM symbols through which the ACK/NAK istransmitted on the downlink may be controlled by cell coverage.Therefore, it is not necessary for the “m” to frequently change for eachcell. Moreover, if the number of OFDM symbols through which the ACK/NAKis transmitted varies per subframe similar to the number of OFDM symbolsfor control channel transmission, it may be difficult to relate theuplink data transmission of each mobile terminal with the ACK/NAKchannels through which the ACK/NAK signals of the data are transmitted.

Therefore, in accordance with the present invention, it is preferablethat the number of OFDM symbols through which the ACK/NAK channel istransmitted (m) vary over a larger period than a period that the numberof OFDM symbols for control channel transmission (n) variesindependently from the number of ACK/NAK signals actually transmitted inan optional subframe. In other words, as shown in FIG. 1, it ispreferable to set the number of OFDM symbols through which the ACK/NAKchannel is transmitted (m) to be relatively semi-static as compared tothe number of OFDM symbols for control channel transmission (n) that canbe variously set per subframe.

Preferably, in order for the mobile terminals to receive the ACK/NAKsignals, an allocation structure of the ACK/NAK channels should be knownso that the base station may notify the mobile terminals of the “m”value through an upper layer RRC message or a broadcast channel with aslower period than the “n” value. Differently therefrom, the “n” valuemay be transmitted per subframe through the CCFI as described above.

FIG. 2 is a diagram illustrating one example of a method for allocatingthe transmission of OFDM symbols of a control channel and ACK/NAKchannels in an orthogonal frequency division multiplexing (OFDM) systemin accordance with one embodiment of the present invention.

In accordance with the present invention, a number of OFDM symbolsthrough which the ACK/NAK channel is transmitted (m) is set as a minimumvalue within a varying range of the number of OFDM symbols for controlchannel transmission (n) that may vary per subframe. Preferably, thenumber “m” of OFDM symbols varies semi-statically. Accordingly, thenumber of OFDM symbols for control channel transmission (n) may beselected among values within a range from the number of OFDM symbolsthrough which the ACK/NAK channel is transmitted (m) to the maximumnumber of OFDM symbols for control channel transmission (N). Thisrelationship is represented by Equation (1).

m≦n≦N  (1)

In Equation (1), “m” represents the number of OFDM symbols through whichthe ACK/NAK channel is transmitted, “n” represents the number of OFDMsymbols for control channel transmission, and “N” represents the maximumnumber of OFDM symbols for control channel transmission. Here, theACK/NAK channel is allocated to first m OFDM symbols. Moreover, like the“N” value, a maximum number of OFDM symbols through which the ACK/NAKchannel is transmitted (M) may be previously determined. Accordingly,the “m” value may be within a range from 0 to M. Preferably, the “M”value is less than or equal to the “N” value.

If the “n” value varies per subframe using the above-described method,and although the amount of time/frequency resources within the “n”number of OFDM symbols capable of being allocated to the ACK/NAK channelin one subframe also varies, the number of OFDM symbols for controlchannel transmission may be varied within a limited range per subframewhile a structure of the ACK/NAK channel is semi-statically fixed inaccordance with one embodiment of the present invention. Examples of thevarying range of the “n” value according to the “M” value will bedescribed with reference to FIG. 2.

FIG. 2( a) is a diagram illustrating an example that the number of OFDMsymbols through which the ACK/NAK channel is transmitted (m) is 1. Inthe example that m=1, the ACK/NAK channel is transmitted throughpredetermined resource elements within a first OFDM symbol of eachsubframe, and the “n” value may vary within a range from 1 to 3 persubframe.

FIG. 2( b) is a diagram illustrating an example that the number of OFDMsymbols through which the ACK/NAK channel is transmitted (m) is 2. Inthe example that m=2, the ACK/NAK channel is transmitted throughpredetermined resource elements within first and second OFDM symbols ofeach subframe, and the “n” value may vary within a range from 2 to 3 persubframe.

FIG. 2( c) is a diagram illustrating an example that the number of OFDMsymbols through which the ACK/NAK channel is transmitted (m) is 3. Inthe example that m=3, the ACK/NAK channel is transmitted throughpredetermined resource elements within first, second and third OFDMsymbols of each subframe. In this particular case, the “n” value isfixed at 3.

Through the above described method, the number of OFDM symbols forcontrol channel transmission may be varied within a limited range persubframe while a structure of the ACK/NAK channel is semi-staticallyfixed, wherein control signals are transmitted on the control channel.Also, if the ACK/NAK channel transmission is performed using the OFDMsymbols for control channel transmission as above, downlink datatransmitted through OFDM symbols other than the OFDM symbols for controlchannel transmission and ACK/NAK signals are multiplexed to betransmitted in each subframe. Accordingly, complication in setting datatransmission power is prevented.

FIG. 3 is a flow chart illustrating one example of a method fortransmitting information on the number of OFDM symbols for controlchannel transmission (n) and a control channel from a base station inaccordance with one embodiment of the present invention.

Initially, a base station may determine the number of OFDM symbols forcontrol channel transmission (n) within a range of minimizing the numberof OFDM symbols through which the ACK/NAK channel is transmitted (m) byconsidering the number of OFDM symbols through which a predeterminedACK/NAK channel is transmitted (S10). Here, the “n” value is preferablyless than or equal to the maximum number of OFDM symbols for controlchannel transmission (N), as described above.

Thereafter, the base station may transmit, to at least one mobileterminal, information regarding the determined number of OFDM symbolsfor control channel transmission (n) (S11). Finally, the relevantcontrol channel may be transmitted to the at least one mobile terminal(S12).

Particularly, when the ACK/NAK channel is allocated to be transmittedthrough the maximum number of OFDM symbols for control channeltransmission (N) that can be used in transmitting scheduling signals(N=M and m=M), as explained with reference to FIG. 2( c), the “n” valuecannot have a value other than n=N. Thus, the “n” value may not bebroadcast through the CCFI per subframe. Accordingly, the time/frequencyresources reserved for CCFI transmission may not be used for CCFItransmission, but may have other uses. Preferably, the time/frequencyresources may be extensively used for control signal transmissionincluding the scheduling signals or the ACK/NAK signals.

In the above descriptions, an “n” value and an “m” value do not alwaysexist in a unit of 1 within n≦N and m≦N, respectively. Rather, thevalues may be selected from a specific natural number set existingwithin n≦N and m≦N. Herein, the specific natural number set may include0.

FIG. 4 is a flow chart illustrating one example of a method forreceiving information on the number of OFDM symbols for control channeltransmission (n) and a control channel in a mobile terminal inaccordance with one embodiment of the present invention.

In the present embodiment, the number of OFDM symbols through which theACK/NAK channel is transmitted (m) is a value that can besemi-statically varied as described above. Preferably, a mobile terminalpreviously acquires information regarding the number of OFDM symbolsthrough which the ACK/NAK channel is transmitted (m) through an upperlayer RRC message or other broadcasting channel before receiving anddecoding a corresponding subframe(s).

In accordance with the present invention, the mobile terminal receivesCCFI, which is information regarding the number of OFDM symbols forcontrol channel transmission (n), through PCFICH. Here, the number ofOFDM symbols for control channel transmission (n) may be varied within arange of minimizing the number of OFDM symbols through which the ACK/NAKchannel is transmitted (m) according to one embodiment of the presentinvention. Preferably, the mobile terminal decodes the received numberof OFDM symbols for control channel transmission (n) by obtainingcorrelation values using expected “n” values that can be the number ofOFDM symbols for control channel transmission, etc.

As stated above, the mobile terminal may assume the expected “n” valuesbased on the “m” value previously informed to the mobile terminalaccording to the present embodiment. Thus, when decoding the “n” value,the mobile terminal may decode the CCFI assuming that the “n” value iswithin the range of m≦n≦N so that the CCFI decoding outputs the “n”value within the range (S20).

After obtaining the “n” value by the above procedure, a mobile terminalmay decode the second control channels assuming the control channels aretransmitted through “n” OFDM symbols (S30).

In another aspect of the invention, the mobile terminal may decode theCCFI to obtain the “n” value without considering the expected range ofm≦n≦N. Therefore, the mobile terminal may obtain the “n” value which isout of the valid range of m≦n≦N. In this case, the mobile terminal maytry to decode control channels for all possible “n” values, or for everypossible “n” value within the range of m≦n≦N.

Otherwise, in another example, when the “n” value obtained deviates fromthe range m≦n≦N, then decoding CCFI is considered to have failed for theparticular “n” value. If so, an operation corresponding thereto may beabandoned. For example, the mobile terminal may abandon receivingscheduling signals in the subframe if the “n” value does not satisfym≦n≦N.

Particularly, as explained with reference to FIG. 2( c), when thealready known “m” is equal to the maximum number of OFDM symbols forcontrol channel transmission (N), such that m=N, then the base stationdoes not transmit the CCFI, or the mobile terminal does not decode theCCFI even though the base station transmits the CCFI because the mobileterminal assumes that n=N. Therefore, the mobile terminal may operateassuming that the scheduling signals and other control signals aretransmitted through the first N OFDM symbols.

Alternatively, if the already known “m” is equal to the maximum numberof OFDM symbols for control channel transmission (N), such that m=N, andif the base station transmits the CCFI, the mobile terminal will decodethe CCFI. However, the mobile terminal will assume that n=N regardlessof the decoding results. Accordingly, the mobile terminal may alsooperate assuming that the scheduling signals and other control signalsare transmitted through the first N OFDM symbols.

FIG. 5 illustrates a block diagram of a mobile station (MS) or UE 1 inaccordance with the present invention. The UE 1 includes a processor (ordigital signal processor) 210, RF module 235, power management module205, antenna 240, battery 255, display 215, keypad 220, memory 230,speaker 245 and microphone 250.

A user enters instructional information, such as a telephone number, forexample, by pushing the buttons of a keypad 220 or by voice activationusing the microphone 250. The microprocessor 210 receives and processesthe instructional information to perform the appropriate function, suchas to dial the telephone number. Operational data may be retrieved fromthe memory module 230 to perform the function. Furthermore, theprocessor 210 may display the instructional and operational informationon the display 215 for the user's reference and convenience.

The processor 210 issues instructional information to the RF module 235,to initiate communication, for example, transmits radio signalscomprising voice communication data. The RF module 235 comprises areceiver and a transmitter to receive and transmit radio signals. Anantenna 240 facilitates the transmission and reception of radio signals.Upon receiving radio signals, the RF module 235 may forward and convertthe signals to baseband frequency for processing by the processor 210.The processed signals would be transformed into audible or readableinformation outputted via the speaker 245, for example. The processor210 also includes the protocols and functions necessary to perform thevarious processes described herein.

FIG. 6 is a diagram explaining an example of a method for receivinginformation of orthogonal frequency division multiplexing (OFDM) symbolsof a downlink control channel in an OFDM system in accordance with oneembodiment of the present invention. Referring to FIG. 6, a mobileterminal receives information about number m of first OFDM symbols whichis used for transmission of a channel, wherein the channel carries ahybrid automatic repeat request (HARQ) ACK/NACK (S61). The mobileterminal receives information about number n of second OFDM symbolswhich is used for transmission of the downlink control channel (S62). Inthis example, the number n is equal to or greater than the number m(n≧m) and a transmission interval of the information about the number mis greater than a transmission interval of the information about thenumber n.

It is obvious that embodiments can be configured by combining the claimsnot having clear citation relations in the claims or new claims may beincluded in the claims by means of amendments after filing anapplication.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or acombination thereof, etc. When implemented by the hardware, a method forreceiving a control channel according to one embodiment of the presentinvention can be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, micro processors, etc.

When implemented by the firmware or the software, a method for receivinga control channel according to one embodiment of the present inventioncan be implemented in the shapes of modules, processes, and functions,etc. performing the functions or the operations explained as above.Software codes are stored in a memory unit, making it possible to bedriven by a processor. The memory unit is positioned inside or outsidethe processor, making it possible to exchange data with the processor bymeans of various means already publicly known.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges might be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art. In theclaims, means-plus-function clauses are intended to cover the structuredescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures.

1. A method of configuring number n of OFDM (Orthogonal FrequencyDivision Multiplexing) symbols which is used for first channels thatcarry downlink control information at a mobile terminal in a wirelesscommunication system, the method comprising: receiving informationindicating number m of OFDM symbols which is used for second channelsthat carry hybrid automatic repeat request (HARQ) ACK/NACKs; and if thenumber m is a predetermined value N, assuming that the number n of OFDMsymbols which is used for first channels is equal to the predeterminedvalue N.
 2. The method of claim 1, wherein the predetermined value N isa maximum number of OFDM symbols for the first channels.
 3. The methodof claim 1, further comprising: receiving the downlink controlinformation and the HARQ ACK/NACKs through the first and secondchannels, respectively.
 4. The method of claim 1, wherein the firstchannels are physical downlink control channels (PDCCHs).
 5. The methodof claim 1, further comprising: receiving information indicating thenumber n of OFDM symbols which is used for first channels, wherein thenumber n is equal to or greater than the number m (n≧m) and atransmission interval of the information indicating the number m isgreater than a transmission interval of the information indicating thenumber n.
 6. A mobile terminal used in a wireless communication system,the mobile terminal comprising: a radio frequency unit; and a processor,wherein the processor is configured to: configure number n of OFDM(Orthogonal Frequency Division Multiplexing) symbols which is used forfirst channels that carry downlink control information; and receiveinformation indicating number m of OFDM symbols which is used for secondchannels that carry hybrid automatic repeat request (HARQ) ACK/NACKs,and wherein if the number m is a predetermined value N, the processor isfurther configured to assume that the number n of OFDM symbols which isused for first channels is equal to the predetermined value N.
 7. Themobile terminal of claim 6, wherein the predetermined value N is amaximum number of OFDM symbols for the first channels.
 8. The mobileterminal of claim 6, wherein the processor is further configured toreceive the downlink control information and the HARQ ACK/NACKs throughthe first and second channels, respectively.
 9. The mobile terminal ofclaim 6, wherein the first channels are physical downlink controlchannels (PDCCHs).
 10. The mobile terminal of claim 6, wherein theprocessor is further configured to receive information indicating thenumber n of OFDM symbols which is used for first channels, wherein thenumber n is equal to or greater than the number m (n≧m) and atransmission interval of the information indicating the number m isgreater than a transmission interval of the information indicating thenumber n.